Portable electronic devices are commonly used for verbal and written communication, scheduling, note taking, etc. A portable electronic device often derives its power from a battery, wherein the battery contains a finite amount of power and must be recharged or replaced when it is depleted. The size of the battery often determines the amount of power it can source, but larger batteries, which can source more power, add considerable weight and size, reducing the convenience of the portable electronic device. For this reason, smaller, less cumbersome batteries are often preferred, but these smaller batteries are often more costly than commercially available large batteries and are limited in their power sourcing capacity.
Considering the limited amount of power that can be sourced by a practically sized and priced battery, reducing power consumption is an important design factor when developing portable electronic devices. If the circuitry in a portable electronic device consumes unnecessary power, or power that doesn't contribute to the functionality of the device, the lifetime of the battery is reduced, which in turn limits the usefulness of the device.
Unnecessary power consumption can be due to leakage current. Leakage current can occur when a device in the circuit is not driven and decays to an unknown, or indeterminate, logic state. In this unknown logic state, the device may be partially "on" thus forming a current path between a supply and a return. As current flows through the path, unnecessary power is consumed.
To eliminate the excess power consumption, the indeterminate states that cause leakage current must be eliminated. While this may sound like a trivial task, it is most complex in an integrated circuit having millions of transistors. In such complex circuits the indeterminate states may occur as a result of propagation delays, parasitic capacitance, etc.
A prior art technique for detecting indeterminate states that cause leakage current on circuits utilizes a low-light or infrared camera to monitor a device that has already been produced. When this technique is used, sites where leakage current is present, i.e. has an indeterminate state, will appear as "hot-spots" or bright areas which can be detected by the camera. These hot-spots are the result of heat generated when power is consumed at the leakage current sites. While the technique identifies leakage current sites, it cannot be performed until after the circuit has already been fabricated, which makes correction of the indeterminate state both time consuming and costly. In addition, as circuits are developed through multi-layer integrated circuit processes or are created on multi-layer boards, it becomes more difficult for cameras to discern which layer the hot-spots are actually on. In addition, the camera method provides little insight as to why the hot spot has occurred.
Another prior art technique involves identifying all of the nodes in a circuit that produce an indeterminate state during simulation. While this technique identifies nodes that produce an indeterminate state, these nodes may not draw any leakage current because they do not produce a conductive path from the supply to the return. In order to identify leakage current sites, the identified nodes must be further analyzed which, for a multi-million transistor integrated circuit, is time consuming and requires post-processing of the indeterminate state information.
Therefore, a need exists for a method that automatically identifies leakage current sites in a circuit prior to fabrication, minimizes the additional time and effort necessary to do so, eliminates the requirement for post-processing, and provides information as to why the leakage current site occurred.